=Intestinal Juice.=—The mucous membrane of the alimentary tract, as far down as the middle of the rectum, is, as previously stated (p. 102), studded with tubular glands. They secrete a light-yellow fluid, alkaline in reaction, and opalescent. Its most important property is due to a ferment which converts cane-sugar into a mixture of dextrose and levulose, and changes maltose—the sugar produced by the action on starch of saliva and pancreatic juice—into dextrose. It is in the form of dextrose that sugar is carried about the body and assimilated by the tissues.
Intestinal juice also contains a ferment, erepsin, which shakes to pieces the heavy molecules of peptones and partly formed peptones. Under its influence they break up into comparatively simple bodies containing the radicle of ammonia. Substances containing an NH₂ group—one H of NH₃ (ammonia) having been given up, in order that the group may have a “free arm” with which to link on to the other component parts of the molecule—are termed “amides.” The amides which are most characteristic of the action of erepsin are leucin, an amidated fatty acid; and tyrosin, an amidated aromatic acid. The tendency of proteins to break up along these two lines—the fatty acid line and the aromatic acid line—is of considerable interest. The one line is represented by acetic acid, CH₃COOH; the other contains the hexone radicle, C₆H₆. Benzoic acid, C₆H₅COOH, is representative of the latter. It used to be thought that proteins which were shaken into simple bodies such as amides were lost to the economy. Their downward career was a foregone conclusion. There could be no arresting it before they brought up at the bottom—as urea, CO(NH₂)₂—the diamide of carbonic acid. It was even supposed that this disintegration of proteins was a provision for getting rid of the surplus animal food which we consume. Physiological chemists now take quite a different view. They believe that the epithelial wall of the intestine through which these substances are absorbed, or the liver, to which they are carried by the portal blood-stream, has the power of recombining these fragments into the complex protein edifice. It is even thought that disintegration is a necessary preliminary to the rearrangement of the sub-groups. A large variety of proteins is ingested as food. Many of them, especially the vegetable proteins, are quite foreign to the body. By the activity of pancreatic juice and erepsin, they are broken into small and relatively stable groups of atoms, which are again fitted together into the particular forms of protein which are of use to the economy.